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Nanophotonics Accessibility and Applicability 4 Potential Military Applications of Nanophotonics INTRODUCTION The Committee on Nanophotonics Accessibility and Applicability believes that there is clearly potential for nanophotonics to have a significant impact on military systems, in both symmetric and asymmetric warfare. In order to assess the potential applications, the committee made reference to the National Research Council’s (NRC’s) report Avoiding Surprise in an Era of Global Technology Advances (NRC, 2005), which describes a framework that is suitable and methodologies that are pertinent for assessing potential applications in the present study. The present committee points out that even as there is clear potential for the significant impact of nanophotonics as described above, it is also likely that nanotechnology insertion into disruptive technologies that could pose threats is difficult to anticipate by the very nature of such threats and the sometimes embryonic state of research in nanophotonics relative to other technologies. Some applications may take substantial investments and time before they are realized. The committee also understands that nanophotonics will require a fabric of supporting and enabling technologies. This technology environment (at least in part) does not exist today, which further complicates the outlook. Within the predefined framework that the committee employed, strong emphasis was placed on information technology (IT) as a possible “game changer,” with nanophotonics being a technology that could potentially enable a much more ubiquitous and pervasive data-processing (and sensing) capability than is currently available. It is also understood that the present study is not intended to promote the potential of nanophotonics, but to make certain that the technology parameters identified are real and can be translated into military applications. Finding 4-1. The committee believes that, because of the infancy of nanophotonics, the probability of near-term revolutionary changes using nanophotonics is small but not negligible, for both domestic and foreign entities.
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Nanophotonics Accessibility and Applicability REPORTING PROCESS AND METHODOLOGY The process and methodology defined in Avoiding Surprise in an Era of Global Technology Advances (NRC, 2005) is being employed in this report (with modifications) for consistency, to convey the response of this committee in a traceable way as it seeks to predict a potential adversary’s military utilization of nanophotonics during a 10-to-15-year time frame (i.e., from 2017 to 2022). This section identifies and describes the methodology that is used to gauge the importance of nanophotonics to the different applications areas. In the next section, “Potential Enabling Technologies and Applications,” the committee members report their assessments of some exemplary applications vignettes using this methodology. Each of the eight such committee assessments on pros, cons, and military risks; and presents recommendations to address the Defense Intelligence Agency’s (DIA’s) concerns about the applications with respect to Joint Vision 2020 (JCS, 2000). As illustrated in the sample Chart 4-1, “Example of Technology Assessment Chart,” the methodology employed addresses the following 10 areas: Application area: area in which a nanophotonics-based technology can be employed. Potential technology: described briefly (in general terms) with respect to the application. Observables: which are indicators of research progress and investments or products derived from nanophotonics, such as publications, funding sources and amounts, and open-market artifacts that the sponsor should monitor to determine growth within the realm of potential military usefulness. CHART 4-1 Example of Technology Assessment Chart Application Area Text and/or graphical representation Technology Observables Brief description of technology Brief description of observables Accessibility Maturity Consequence Level 1, 2, or 3 Technology Futures Technology Watch Technology Warning Technology Alert Short characterization of consequences Enablers and Key Technical Parameters Put the technology into a lay person’s perspective Triggers List new developments to watch that may enable warfare capability. Narrative(s) Summarize current research and development. Assessment Summary Include pros, cons, overall view, and military utilization, where available.
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Nanophotonics Accessibility and Applicability Three levels of accessibility of the technology to allies or adversaries in terms as stated in the NRC’s Avoiding Surprise in an Era of Global Technology Advances (NRC, 2005, p. 25): The accessibility variable focuses on the question, How difficult would it be for an adversary to exploit the technology? It addresses the ability of an adversary to gain access to and exploit a given technology. This assessment is divided into three levels: Level 1. The technology is available through the Internet, being a commercial off-the-shelf item; low sophistication is required to exploit it. Level 2. The technology would require a small investment (hundreds of dollars to a few hundred thousand dollars) in facilities and/or expertise. Level 3. The technology would require a major investment (millions to billions of dollars) in facilities and/or expertise. In general, Level 1 technologies are those driven by the global commercial technology environment; they are available for exploitation by a diverse range of potential adversaries. Level 3 technologies, by contrast, are typically accessible only to state-based-actors. The indicators likely to be of value in determining an adversary’s actual access to a given technology vary by level as well as by the type of technology. Maturity of the technology in terms of its readiness level, as expressed in Avoiding Surprise in an Era of Global Technology Advances by Futures, Watch, Warning, and Alert (NRC, 2005, p. 25): The maturity variable focuses on the question, How much is known about an adversary’s intentions to exploit the technology? It integrates what is known about an adversary’s actions, together with an evaluation of the state of play with respect to the technology of interest. At the highest level, called Technology Alert, an adversary has been identified and an operational capability has been observed. At the lowest level, Technology Futures, the potential for a technology-based threat has been identified, but no positive indicators have been observed. The Maturity assessment is divided into four categories: the first two (the lower levels) suggest further actions for the technology warning community; the other two indicate the need for immediate attention by military leadership: Futures. Create a technology roadmap and forecast; identify potential observables to aid in the tracking of technological advances. Technology Watch. Monitor (global) communications and publications for breakthroughs and integrations. Technology Warning. Positive observables indicate that a prototype has been achieved. Technology Alert. An adversary has been identified and operational capability is known to exist. Strategic consequences to the sponsor if this technology development area is not monitored, viewed from a defensive military perspective as defined in Avoiding Surprise in an Era of Global Technology Advances (NRC, 2005, p. 26): Characterization of a technology in terms of the consequence variable involves addressing the question, What is the impact on military capability should the technology be employed by an adversary? It involves assessing the impact of the postulated RED technology on the capability of BLUE forces. This impact can range from denial or negation of a critical capability to the less-consequential level of annoyance or nuisance. A corollary assessment may be made as to the locus of impact—that is, whether the technology affects a single person, as in the case of an assassination, or creates a circumstance of mass casualty and attendant mass chaos.
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Nanophotonics Accessibility and Applicability Technology enablers and key technical parameters, including packaging and interface requirements, described in lay terms. Triggers, which may be unpredictable but which are nonetheless important in enabling significant revolutionary developments, particularly those deemed disruptive. A narrative in which the current research and development (R&D) is summarized and where the leaders in the R&D may be identified by way of sources in the current literature. An assessment summary that includes the assessment of the technology in terms of “pros, cons, overall view, potential military utilization and associated risks.” Graphic representations, such as photographic and other images, of the potential technology and its associated application are provided as appropriate to the particular applications. POTENTIAL ENABLING TECHNOLOGIES AND APPLICATIONS The committee deliberated on the potential technologies and applications that nanophotonics can enable, as well as the infrastructure needed to enable the nanophotonics base technology in the first place. Key aspects of this infrastructure are described in Chapter 3. Several general, militarily relevant applications categories and their respective specific subcategories are introduced below. The specific subcategories, shown in italics, are described in the example vignettes in Charts 4-2 through Chart 4-9. Although the general categories reflect a military emphasis, many of the specific applications have commercial and space utility as well. The general categories are sensing; command, control, and communications; computing; countermeasures; and power. Sensing is the largest of these; its subcategories include imaging sensors (including night vision as well as long wave and mid-wave infrared sensors), chemical sensors, biosensing (including biometrics), and advanced spectroscopy for sensing and materials characterization. Other more mission-specific areas in which nanophotonics devices have been envisioned include health monitoring, tagging, tracking, locating, or eavesdropping, and situational awareness in general. In the general category of command, control, and communications, applications can be enabled for a variety of potential needs, such as platform avionics, the remote actuation of devices, as well as emissive displays for monitoring. The field of secure communications, including quantum key distribution, is also potentially fertile ground for nanophotonics insertion. Nanophotonics applications in the category of computing are especially anticipated in robust computation, computing systems and microprocessors, data storage, and optical signal processing. This study’s use of the term “countermeasures”—another general category—is actually rather broad. It spans active countermeasures and stealth but also includes sensor protection, bioremediation, and decontamination. Promising applications in the power category include micropower, solar cells, and energy harvesting. Finally, in the section “Technologies in Their Infancy,” a more detailed look at the state-of-the-art of quantum computing and nanophotonics is provided, and the adjunct field of terahertz spectroscopy and nanophotonics is also described.
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Nanophotonics Accessibility and Applicability CHART 4-2 Application Area: Solar Cells and Micropower FIGURE 4-2-1 Efficient, lightweight photovoltaic cells provide battery rechargers, tents and shelters, and other electrical devices for the soldier in the field. SOURCE: Image provided by Steve Smith, SAIC. Reprinted with permission. Technology Observables Efficient absorption of solar energy and conversion of photons to electronic carriers to provide power to drive devices. Incorporation of nanophotonic materials or phenomena to enhance photovoltaic efficiencies. Research and development activities related to nanoscale photovoltaic devices. Start-up companies in this area. Accessibility Maturity Consequence Level 3 Technology Watch Pervasive, lightweight sources of power would enable continual access to computation and communications, facilitating the capability to pursue military goals.
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Nanophotonics Accessibility and Applicability Enablers and Key Technical Parameters Single-crystal silicon solar cells comprise the dominant photovoltaic (PV) technology, with module efficiencies of about 10 to 12 percent and module costs of about $3.50 per watt. Solar power costs, even with a subsidy, are not competitive with more traditional energy sources; this has prevented solar power’s more widespread commercial appearance. Cost may not be the dominant issue for military or other specialized applications, where high energy conversion efficiency and the existence of lightweight, renewable energy sources may be of paramount importance. The single bandgap (single junction) cell limits the total range of the solar spectrum that can be captured, thus limiting the efficiency, while the single-crystal substrate provides constraints to achieving lower cost and manufacturability of large-area solar cells. Multiple-junction solar cells (e.g., the gallium indium phosphide/gallium arsenide/germanium [GaInP/GaAs/Ge] cell developed by the National Renewable Energy Laboratory [NREL] and Spectrolab) with semiconductors that collectively span a greater fraction of the solar spectrum have shown efficiencies well in excess of 30 percent, but the epitaxial or pseudomorphic matching of the layers and the critical tolerances on device design set limits to cost and manufacturability. Key innovations in improving the efficiency of solar cells while maintaining reduced costs focus on (1) creating low-cost absorbers that will fully capture the full range of the solar spectrum, (2) maximizing solar absorbance while not compromising electron-hole formation, separation, and collection. In recent years, engineering of nanoscale components has offered new approaches for PV enhancement. Regular arrays of nanocrystals (quantum dots) are predicted to form mini-bands or “multiple energy level” solar cells that more efficiently convert absorbed photons to electrical carriers, rather than creating dissipated heat. Plasmonic structures could serve as antennas or amplifiers that could resonantly couple photons to the PV material (Lewis et al., 2005). Triggers Potential revolutionary opportunity: Portable, lightweight sources of power for the battlefield. Narrative(s) Two new solar cell companies cited as offering nanostructured PV materials are Konarka and Nanosolar. FIGURE 4-2-2 Konarka’s flexible solar cells. SOURCE: Image courtesy of Konarka Technologies, Inc. Reprinted with permission.
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Nanophotonics Accessibility and Applicability The first of these, Konarka, was founded in July 2001 and focused on the development and advancement of nano-enabled polymer photovoltaic materials that are lightweight, flexible, and more versatile than traditional solar materials. Konarka’s technology represents a new breed of coatable plastic flexible photovoltaic material that can be used in many applications where traditional photovoltaics cannot compete. The resulting plastic-based photovoltaic cells (see Figure 4-2-2) are efficient across a much broader spectrum of light than traditional solar cells. The second company, Nanosolar, founded in 2002, incorporates nanostructured material components as the basis of large-area, printable photovoltaic structures (see Figure 4-2-3). FIGURE 4-2-3 Nanostructured and roll-to-roll processed materials. SOURCE: Image courtesy of Nanosolar, Inc. Reprinted with permission. Assessment Summary Pro(s): Much of this technology requires long-range development, and there are many open scientific and technological questions. Plasmonic antennas and amplifiers integrated into semiconductor structures may instead promote optical loss. It may not be possible to achieve the requisite control of nanocrystal size and placement to produce multiple-energy-level solar cells. Con(s): Large-scale development and deployment of photovoltaics may be accelerated by other countries regardless of initial issues of cost. Alternative systems-level strategies may produce higher-efficiency, compact photovoltaics, without requiring nanophotonic concepts. Overall View: The committee views this as a Technology Watch item. Various nanophotonics-related ideas are being pursued to enhance photovoltaic performance, but each approach currently faces important scientific and technology uncertainties. The committee believes that this area should be (1) reassessed yearly and/or that (2) a database should be employed to obtain real-time critical information from the science and technology (S&T) communities involved in nanophotonics R&D efforts. Military Application Considerations/Suggested Risks: Lightweight, portable sources of energy for the battlefield are of paramount importance. In 2006, tens of millions of dollars were spent on batteries alone for deployment in the battlefield (Bennett, 2007). It is not only the savings in funds but the reduction in bulk and weight and the reliability of power that would be a critical enabler for the soldier. In addition, there are obvious, enormous commercial benefits in realizing highly efficient, low-cost solar energy sources.
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Nanophotonics Accessibility and Applicability CHART 4-3 Application Area: Remote Sensing FIGURE 4-3-1 Nanostructured functional device fibers. SOURCE: Bayindir et al. (2006). Reprinted with permission. © 2006 IEEE. Technology Observables Remote sensing uses a device to gather information about the environment from a distance. Nanophotonics can sense a wide range of chemicals, (e.g., explosives) and organisms (e.g., viruses) as well as physical effects on materials (temperature, strain, vibrations, etc.) because of its noninvasive nature. Photonic nanostructures can be integrated into a variety of different media: fibers, nanocomposites, fabrics, and so on. Optical methods can provide both sources and detector sensors. Prior work on silica and polymer optical fibers arranged to create two-dimensional air/material photonic crystal fibers demonstrated useful materials as optical waveguides and sensors. Multimaterial fiber sensors are linear versions of the familiar finite, essentially two-dimensional planar devices incorporating insulating, semiconducting, and metallic elements that are used in traditional electronic, optoelectronic, and thermal detection devices. The approach is to prepare a preform and to thermally draw the preform into fine-diameter, kilometer-long fibers (see panel e in Figure 4-3-1). Sensing is a trade-off between sensitivity, selectivity, response time, and cost. A directed beam of light can excite characteristic optical response from the analyte(s), which may be detected back at the source location. Difficulties are that the spectral response from laser excitation (e.g., infrared or Raman spectroscopy) is poor at the most useful distances, and critical emissions may be absorbed or scattered by the path medium between the source (agent) and the detector due to the long path lengths involved. Alternatively, a set of sensors can be placed in an environment, and these devices can locally detect changes resulting from interaction with the analyte(s) and then send the information back to a base wirelessly, or be interrogated by a laser excitation or use the change in the optical properties of the device by the interaction with the analyte(s) to modulate a lasing output from the device. The multimaterial fibers can be made into a very large area cross-array that offers N2 local detectors while requiring only 2N I/O (input/ output) readers.
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Nanophotonics Accessibility and Applicability Accessibility Maturity Consequence Level 3 Technology Watch These fibers have been under development only recently and in a university setting, but the know-how of fiber drawing is well established in the fiber telecommunications industry. The multimaterial preform approach allows rapid exploration of device designs at millimeter scale, that are demagnified thousands of times during the drawing process. Enablers and Key Technical Parameters Wearable devices that can be integrated (by weaving the fibers into a fabric) into common items that people wear or carry (e.g., clothes, knapsack, etc.) could make every person a mobile sensor platform. Additionally, fibers can be fabricated into simple flat arrays and attached to buildings or vehicles. Because they are linear and can be shaped into two- and three-dimensional arrays, they are no longer point detectors but area and volume detectors. Use of a photoconductive material would make the fibers responsive to light; placement of concentric filters around the photoconductive core makes the fibers able to “see” different colors of light. There are a host of known ways to make the optical characteristic of materials change due to the presence of a substance (e.g., chemical or explosive agent), and thus these fibers can sense their environment and either broadcast their information or be interrogated from a distance. The fibers can have a central hollow core and thus can hold coatings for binding analytes that would change the optical properties of the fibers (see Figure 4-3-2 in the “Narrative” section below). Triggers Potential revolutionary opportunities: Very large arrays that are ultralightweight (due to open fiber constructs) and N2 detectors for 2N fibers. Warfighter-wearable, light unmanned aerial vehicle (UAV) payload, skins of buildings, aircraft, ships for full area coverage. Low power consumption.
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Nanophotonics Accessibility and Applicability Narrative FIGURE 4-3-2 (a) Hollow core fiber; (b) results of optical and electrical measurements through fabric; (c) spectrometric fabric. SOURCES: Reprinted by permission of Macmillan Publishers Ltd: Nature, Bayindir et al. (2004); Venema (2004); Luan et al. (2004). Reprinted with permission. © 2004 IEEE. Assessment Summary Pro(s): Nothing like this is available for creating large-area or volume sensing. Con(s): Only the Massachusetts Institute of Technology group has done the multi-material fiber work; the University of Bath, United Kingdom, leads in the two-dimensional photonic crystal fiber work. The Danish company Crystal Fibre A/S produces and markets the two-dimensional photonic crystal fibers. Overall View: The committee views this technology as a Technology Watch item. Companies that can do fiber drawing (e.g., telecommunications fibers [Corning, etc.]) would be able to commercially produce these fibers. Military Application Considerations/Suggested Risks: Warfighter-wearable, light UAV payload, skins of buildings, aircraft, ships for full area coverage. Robustness against environment, stresses, and high and low temperatures needs to be assessed.
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Nanophotonics Accessibility and Applicability CHART 4-4 Application Area: Emissive Displays FIGURE 4-4-1 Examples of emissive displays. SOURCE: (left and right) Images courtesy of the U.S. Army; (middle) L-3 Communications (2007). Reprinted with permission. Technology Observables The technology has two nanophotonic components: The use of nanoparticles as emitters, and The use of photonic band-gap structures. Periodic dielectric structures can be used to control bandwidth and directionality of light emitted by a variety of sources. These structures can act as large-area vertically emitting lasers, which do not require an external cavity. The structures can be mechanically flexible. The structures can be electrically, mechanically, or optically tunable. Band-gap structures can be efficiently formed using self-assembly. They are ideally suited for emissive display applications. Photonic band-gap structures exhibit a strong reflection band over a range of wavelengths. The spectrum of light emitted by sources in band-gap materials is altered; emission is suppressed in the band and enhanced at the band edges. The direction of emitted light is modified; emission is enhanced along the crystal axes. Photonic bandgap structures can be used to modify light emitted by conventional phosphors, as well as by fluorescent, electro-luminescent, light-emitting diode, and other light sources. If a gain medium is present, band-gap materials can exhibit low-threshold mirrorless lasing due to distributed feedback. The optical properties of band-gap structures can be tuned (via strain, temperature, fields, etc.). They can be used as Agile active filters for sensor (eye) protection, Tunable laser sources, and Reflective elements in transflexive displays.
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Nanophotonics Accessibility and Applicability Technology Observables The infrared (IR) spectral region is of particular importance to military systems. Night vision typically refers to high-gain systems (photomultipliers and avalanche photodiodes) that amplify and detect near-IR ambient radiation, e.g., from 780 nm to almost 3 µm. Both 3 µm to 5 µm mid-wave infrared (MWIR) and 8 µm to 12 µm long-wave infrared (LWIR) spectral regions are of critical importance for thermal and spectroscopic detection, especially in surveillance systems, weapons systems guidance, and missile seekers. The best MWIR and LWIR imagers are typically cooled semiconductor focal plane arrays such as indium antimoride (InSb) and mercury cadmium telluride (HgCdTe), which are used in expensive platforms but are not suitable for human-carried applications. Microbolometer arrays offer 300 K operation, but at lower sensitivity and several-orders-of-magnitude lower speed than their cooled semiconductor counterparts. Nanophotonics offers for the first time the potential of bringing the performance comparable to cooled semiconductor focal plane arrays to operation at room temperature (and thus human-portable operation). This would be a significant development that would change the nature of warfare by providing more detailed information at lower levels of operation. In particular, plasmonic structures can be used to concentrate IR energy striking a pixel area in the focal plane array to a much smaller detector area that can be subwavelength in extent. Since 300 K IR semiconductor detectors are volumetric leakage (dark) current limited, the sensitivity is improved to the extent that the detector volume is reduced while still collecting the signal. The figure of merit is where η is the intrinsic quantum efficiency and the square root of the areas arises because the noise scales as the ½ power of the dark current. The speed scales as Apixel/Adetector as a result of the reduced capacitance. Actual implementation will also require lenslet array registration with the small detector area to maximize optical throughput. Accessibility Maturity Consequence Level 1 Technology Futures Greatly enhanced capabilities for thermal detection on the battlefield, increased situational awareness at lower levels of the command chain, vulnerability of large-platform assets. Enablers and Key Technical Parameters Not applicable. Triggers Potential revolutionary opportunities: Research developments; Reports of field concentration in scales less than a wavelength; and Advances in nonlinear optical processes resulting from similar field concentrations in photonic and metamaterial experiments. Narrative(s) Not applicable.
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Nanophotonics Accessibility and Applicability Assessment Summary Pro(s): The potential is enormous. Con(s): The technology is immature, with active research programs under way at the basic research level. Overall View: The committee views this as a Technology Futures item because the technology is in the research stage and in the open literature. The trigger will be when the research is no longer reported openly; the question will be—Is that because results are no longer forthcoming, or because the results are so important that military secrecy prevails?
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Nanophotonics Accessibility and Applicability CHART 4-9 Application Area: Nano-Enabled Advanced Spectroscopies for Chemical-Biological Threat Sensing (e.g., Surface-Enhanced Raman Spectroscopy) FIGURE 4-9-1 A selective and sensitive detection method for chemical and biological analyte molecules. NOTE: SERS, surface-enhanced spectroscopy. SOURCE: (top left) Schuck et al. (2005); (bottom left) Willets and Van Duyne (2007); (top and bottom right) Halas Research Group, Rice University (2007). Reproduced with permission. FIGURE 4-9-2 Possible detection platform installed on unmanned ground vehicle for the detection of chemical/biological agents. SOURCE: (robot) Northrop Grumman Corporation (2007); (left top and bottom) Hsing-Lin Wang, Los Alamos National Laboratory (2007). Reproduced with permission.
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Nanophotonics Accessibility and Applicability Technology Observables The Raman spectrum of each molecule is a “fingerprint” that uniquely identifies the molecule. Surface plasmons have been used to enhance the surface sensitivity of these techniques, and the effects can be further enhanced using nanostructured metals such as silver (Ag) and gold (Au). The synthesis and growth of these Ag and Au nanoparticles are well established in the literature, including innovative new types of nanostructures that provide further enhancements. With the development of special substrates using plasmonic nanoparticles, enhancements of the Raman signal on the order of 109 are easily attainable. This allows for specific detection of very low concentrations of the analyte molecules using surface-enhanced Raman spectroscopy (SERS). The large enhancement factor for SERS enables the detection of molecules with extremely weak Raman cross sections. SERS detection is the most mature of the various surface-enhanced spectroscopies. Currently, detection of simulants of bioterrorist agents, such as Bacillus subtilis (a harmless simulant of Bacillus anthracis) and half-mustard gas has been reported using SERS. Laser systems for SERS are getting cheaper and smaller, with the development of solid-state lasers. These technologies have been developed comparatively recently and offer an attractive route for continued cost and size reduction over time. There is a need to develop a larger library of “fingerprints” of molecules and compounds of interest. Accessibility Maturity Consequences Level 1 Technology Alert This technology is already everywhere in the literature. It has grown tremendously in the past 8 to 10 years. This technology is currently being studied in many countries, including Japan, China, Russia, and Europe.
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Nanophotonics Accessibility and Applicability Enablers and Key Technical Parameters Raman scattering is the inelastic scattering of light by molecules. The number of photons that undergo Raman scattering is very small. Thus, traditional Raman scattering is observed for few molecules that have a large Raman scattering cross section. Raman spectroscopy is a well-known technique that has been around for decades. The observation of large enhancements in the signal when the molecules are adsorbed onto, or brought in close proximity to, specially designed substrates, has led to the recent interest in SERS as a technique for sensitive and rapid detection of molecules. The large enhancement also allows for detection of molecules with weak Raman scattering cross sections and low concentrations of the analyte molecules. Metal nanoparticles of various shapes and geometries have been employed successfully as SERS substrates. Metal nanoparticles support surface plasmons (a collective oscillation of the electrons in the metal), which leads to an enhancement of the electric field near the surface of the nanoparticles and in the small junctions between nanoparticles. Molecules adsorbed onto the surface of the nanoparticles, or in close proximity to the nanoparticle surface, experience this enhanced electric field, which in turn leads to an enhancement in the SERS scattering by the molecules. Substrate development for use with various laser systems from the visible to the near infrared has been achieved. SERS using near-infrared lasers is of interest, as near-infrared light can penetrate human tissue without causing damage to the tissue. This allows for in vivo sensing using SERS. The SERS signal and the number of molecules available for interrogation determine the resultant SERS signals. An important related nanophotonics technology involves surface plasmon resonance (SPR) reflectivity measurements, surface-sensitive, spectroscopic methods that can be used to characterize the thickness and/or index of refraction of ultrathin organic and biopolymer films at noble metal (Au, Ag, copper) surfaces. SPR spectroscopy has become widely used in the fields of chemistry and biochemistry to characterize biological surfaces and to monitor binding events. The success of these SPR measurements is primarily due to three factors: (1) with SPR spectroscopy the kinetics of biomolecular interactions can be measured in real time, (2) the adsorption of unlabeled analyte molecules to the surface can be monitored, and (3) SPR has a high degree of surface sensitivity that allows weakly bound interactions to be monitored in the presence of excess solution species. SPR spectroscopy has been used to monitor such events as antibody-antigen binding, deoxyribonucleic acid (DNA) hybridization, and protein-DNA interactions. Use of nanoparticle arrays functionalized with antibodies enables the bio and biomedical interactions. Triggers Potential revolutionary opportunities: Development of SERS spectra library of molecules of interest such as chemical and biological hazards; Ruggedized packages; Low-cost manufacturability: compact tunable photon sources for asymmetric warfare; Hand-held (portable): lightweight, low power, miniaturized packages; and Deployable units: packaging suitable for extreme environments and light-on-a-chip integrated systems.
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Nanophotonics Accessibility and Applicability Narrative(s) The example cited below of an enabling application is readily available in the public domain, which makes extending research and development in this area unpredictable (with unrealized and unbounded applications), and subsequently should be reinvestigated at least every 6 months. Example: Stable SERS substrates for anthrax biomarker detection. (See Figure 4-9-3). In 2006 it was reported in the literature that a modified SERS substrate with a shelf life greater than 9 months had been developed. The SERS signal from calcium dipicolinate (extracted from Bacillus subtilis, a harmless stimulant of Bacillus anthracis), a biomarker for anthrax, was measured. A 10 second data collection time is capable of achieving a limit of detection of 1.4 × 103 spores. These substrates demonstrate twice the sensitivity with 6 times shorter data-acquisition time and 7 times longer temporal stability. The modifications proposed expand the palette of available chemical methods to functionalize SERS substrates, which will enable improved and diverse chemical control over the nature of analyte-surface binding for biomedical, homeland security, and environmental applications. FIGURE 4-9-3 (Left) A silver film over nanosphere surface-enhanced Raman spectroscopy (SERS) substrate modified with a layer of alumina. (Right) (A) SERS spectrum of the anthrax biomarker on the unmodified SERS substrate; (B) the same on the alumina modified substrate. SOURCE: Reprinted with permission from Zhang et al. (2006). © 2006 American Chemical Society. Assessment Summary Pro(s): The technology is rapidly maturing, and the United States is a leader in this field. Con(s): The technology is maturing; however, foreign countries (Japan, China, and Europe) are equally strong in this area. Most of the leading instrumentation and commercially available SERS substrates are manufactured by European countries. Overall View: The committee views this technology as a Technology Alert item because the technology is rapidly maturing in many countries. The committee believes that this area (1) should be reassessed every 6 months and/or that (2) a database should be employed to obtain real-time critical information from the science and technology communities involved in nanophotonics R&D efforts. Military Application Considerations/Suggested Risks: The application (depicted in this chart) is an example of an enabling capability that can be leveraged for military warfare both symmetrically and asymmetrically. The following risks have been assessed by the committee to indicate where it believes the state of applications is with respect to the phenomenology. This technology allows for sensitive detection of materials (possibly harmful agents in the field or at specific sites) before personnel are exposed to them. Situational Awareness: Risk: High—Technology allows for advanced analysis of materials enabling chemical/biological sensing. Nanophotonics offers opportunities for concentrating optical energy on scales of less than the wavelength. The chemical/biological sensing capability is enabled because the resulting large fields will lead to increased importance of nonlinear material response.
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Nanophotonics Accessibility and Applicability TECHNOLOGIES IN THEIR INFANCY Quantum Computation and Nanophotonics Mapping computation onto quantum mechanical rather than classical elements affords the possibility of far more powerful computational schemes. Feynman was among the first to explore quantum computation and the possibility of computer algorithms that could be more efficiently processed by quantum systems than by classical systems (Feynman, 1982, 1984, 1986). Deutsch provided some initial examples of such algorithms, and also demonstrated how any physical process, in principle, could be modeled perfectly by a quantum computer (Deutsch, 1985). A key advance then came with Shor’s recognition that an important problem, the determination of prime factors, could be solved in exponentially less time using a processor based on quantum systems, that is, a quantum computer (Shor, 1994). Grover soon developed another important algorithm for quantum computers, showing increased speed in searching through unsorted databases (Grover, 1997). In the same time frame, related developments explored the unique properties of quantum systems in enhancing secure communications. In 1984, Bennett and Brassard introduced the idea of quantum key distribution (QKD), a method for the secure distribution of a cryptographic key (Bennett and Brassard, 1984). In addition to more secure communications, quantum dense coding would allow communications at far higher rates than those possible with classical communications (Bennett and Wiesner, 1992). The compelling potential of quantum computation and communications has recently led to a plethora of research efforts to provide the experimental verification of key quantum information concepts. In quantum computation, the quantum bit, or qubit, replaces the bit as the unit of computation, and in principle, any two-level system that behaves quantum mechanically can serve as a qubit. Recent experimental explorations have used discrete atomic states, the spin states of an electron, and superconducting states as the basis for quantum computation. Elementary quantum logical operations can then correspond to controlled transitions between the states of a qubit. Scaling up the size and complexity of a quantum system brings additional challenges, and some early candidates for scalable systems have included linear arrays of ions within an ion trap, or collections of nuclear spins that are addressed and controlled using the methods of nuclear magnetic resonance (Cirac and Zoller, 1995; Gershenfeld and Chuang, 1997). Superconducting systems, with macroscopic quantum effects, have also served as the basis of quantum computation, and on February 13, 2007, D-Wave Systems, Inc., a Canada-based company, announced a 16-qubit superconducting adiabatic quantum computing processor (D-Wave Systems, 2007).1 In evaluating these and other possible physical bases for quantum computers, DiVincenzo (2000) provided a useful, widely cited guideline, comprising five criteria: The system should be a scalable physical system with well-defined qubits. It should be initializable to a simple reference state such as 000…>. The system should have long decoherence times. It should have a universal set of quantum gates. It should permit high-quantum-efficiency, qubit-specific measurements. 1 The committee did not have time to conduct a thorough quality assessment of these different products. The committee mentions these products as a random sampling of possible applications. The companies listed are pioneers in these areas and have only recently been formed. It is too early to fully assess the quality of their products: that determination will be made as their products are more widely used. The D-Wave Systems product is still rather controversial and under evaluation. Experts in the field were called in to assess the performance of the D-Wave System computer, which is based on “superconducting qubits.” The committee considered it important to note that even in the very demanding and futuristic area of quantum information processing, technological progress is such that products are being generated and introduced into the commercial sector.
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Nanophotonics Accessibility and Applicability Nanophotonic systems can be critical in creating the critical elements of a quantum computation scheme that is either based on well-defined photon states (with different states of polarization comprising the different qubit states) or mediated by the controlled transmission of photons between qubits. Quantum dot environments can localize charged carriers, excitons, or individual electron spins, increasing coherence lifetimes. High-Q nanocavities can form a lossless, well-isolated environment for qubits (minimizing decoherence), with spatially remote qubit interactions determined through the engineered coupling of photons to well-defined modes of the nanocavity. Therefore, the combination of quantum dots within high-Q nanocavities can prove exceptional testbeds for the development of quantum computation strategies (Imamoglu et al., 1999). In addition, correctly engineered nanophotonic cavities may produce an efficient means of creating and manipulating entangled photon (polarization) states (Irvine et al., 2005, 2006). Beyond computation, quantum information technology holds direct benefits to the technology of secure communications, also referred to as quantum key distribution or quantum cryptography (Bennett and Brassard, 1984). The advantages of a quantum information system in detecting eavesdroppers between sender and receiver lie in the essential quantum mechanical property of a state: once measured, the state itself is altered. The first experimental demonstration of QKD was carried out in 1991 by Bennett and co-workers, with transmission of information over a distance of 32 centimeters (Bennett et al., 1991). Within 10 years, subsequent free-space and fiber-enabled experiments demonstrated secure transmission over distances of tens of kilometers (Hughes et al., 2000; Stucki et al., 2002). None of these systems explicitly depended on the implementation of nanophotonics; however, a critical enabling technology for this and other computation and communications applications are true, controlled single photon sources. Some of the successful recent approaches for the formation of such sources rely on elements such as quantum dots, embedded within, and controlled through the mediation of a high-Q nanocavity (Michler et al., 2000; Yamamoto, 2006). In many regards, the notion of accessing and taking advantage of the quantum nature of matter should not be a surprising one, given the manifestation of quantum mechanical behavior at sufficiently small spatial scales. Primary challenges have been in (1) determining the application areas of clear benefit for quantum computing and quantum information processing, (2) constructing realistic (solid-state) quantum systems with minimal decoherence, and (3) determining means of scaling such systems up to sizes and complexity (e.g., number of qubits) to be able to accomplish appropriate computation or communications tasks. With regard to the existence of compelling applications, substantial impetus to the field of quantum computation was given by Shor’s prime factoring algorithm (Shor, 1994). In the case of quantum cryptography, a scant 7 years transpired between Bennett and Brassard’s introduction of quantum key distribution in 1984 and the first experimental demonstration of a secure communications system in 1991 (Bennett et al., 1991; Bennett and Brassard, 1984). The physical realization of secure photon-based communications has been less challenging than the realization of a quantum computer, where the issues of decoherence (and error correction) pose formidable challenges. Basically, the fidelity of a quantum mechanical state must have coherence lifetimes well in excess of typical computation times (nanoseconds or less). With all of these challenges, many researchers have in recent years demonstrated coherence of atomic, ion, spin, and photon states. Further evaluation must be made of D-Wave Systems’ 16-qubit quantum processor, but such an announcement presages only in small part the possibilities of the future. It is still too early to predict the best physical implementations of quantum computation, but many aspects of nanophotonics are expected to play critical enabling roles in these areas.
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Nanophotonics Accessibility and Applicability Terahertz Spectroscopy and Nanophotonics Terahertz may be the best frequency range in the electromagnetic spectrum for high-confidence, high-specificity detection of chemicals in the vapor phase. This is because many molecules, from simple diatomic chemicals to complex macromolecules have stronger and more distinctive absorption and emission resonances in the terahertz range than in either microwave or near-infrared-to-visible ranges (Siegel, 2002; Woolard et al., 2002).2 At lower pressures, smaller molecules generally have very sharp terahertz signatures, with Doppler limited widths around 1 megahertz (MHz), providing significantly enhanced spectral resolution as compared with that of infrared signatures. While compared with microwave spectroscopy, interaction strengths are generally larger in the terahertz regime since the strength of the interaction increases by greater than the square of the frequency, with a peak located in the terahertz regime that is specified by the molecular mass. Nearly all non-centrosymmetric molecules have resonances between 0.1 THz to 10 THz that are predominately rotational modes or hybrid rotational-vibrational modes, determined by a molecule’s moment of inertia. Since the moment of inertia depends on the distribution of mass within the molecule as well as on total mass, a spectrum based on moment of inertia will discriminate between molecular species better than any form of spectroscopy based only on mass. In fact, at high spectral resolution, each molecular signature is unique enough that only a few lines are generally needed to identify a molecule, which is important, given the atmospheric propagation properties described below. An advantage of terahertz spectroscopy is that analysis of the rotational constants can be performed using fundamental quantum mechanics theories. With these measured rotational constants, one has an absolute identification of the molecule quantitatively as well as qualitatively. In most cases of interest, molecular absorption/emission cross-sections are also very large in the terahertz range, leading to potentially excellent detection signal-to-noise levels and thus high sensitivity to small concentrations. For most environments, the thermal energy available exceeds the rotational energy transitions; thus, molecules will emit as well as absorb at the rotational transitions. Thus, for gas molecules that are at a higher temperature than that of the background, the characteristic spectral features will be observable passively as well as actively. The most mature terahertz application is remote sensing by atmospheric scientists and astrophysicists. Thus, a large database of terahertz signatures of most atmospheric constituents already exists, reducing measurement uncertainties by providing potential background signals for a real measurement. Efforts by these communities have demonstrated the strength of the terahertz spectrum and provided a solid foundation on which to expand. These basic physical facts mean that the terahertz regime has enormous potential in the area of remote spectroscopy, with unprecedented, unsurpassed species-discrimination capability and a minimized probability of error due to either missed detection or misidentification. Success in developing the terahertz regime for remote vapor detection will create a new modality in remote sensing that stretches frequency agility, complements conventional microwave and infrared detection by providing hitherto inaccessible primary and corroborative spectral information, and decreases operational predictability by deploying a new and unconventional frequency technology that will make counterdetection and interdiction more difficult. 2 Probably the most complete information on molecular resonances for a large number of common molecules, at frequencies from microwave through ultraviolet, is compiled in the high-resolution transmission molecular absorption database currently maintained by the Harvard-Smithsonian Center for Astrophysics. The database is accessible from the Web site http://cfa-www.harvard.edu/hitran//.
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Nanophotonics Accessibility and Applicability RECOMMENDATION Recommendation 4-1. To enable a more efficient technology watch and warning process for the U.S. intelligence community, the committee recommends that a data-mining tool be developed to uncover “triggers” and “observables” that will enable the U.S. national security establishment to preserve the dominance of the nation’s warfighting capability. In order to uncover pertinent information, the U.S. government could provide a mechanism to leverage critical information from the nanophotonics community. Such a secure and structured database could reveal (across all of the military services) technologies that can support multiple service needs, while also stimulating domestic nanophotonics developments. REFERENCES Bayindir, Mehmet, Ayman F. Abouraddy, Ofer Shapira, Jeff Viens, Dursen Saygin-Hinczewski, Fabien Sroin, Jerimy Arnold, John D. Joannopoulos, and Yoel Fink. 2006. Kilometer-long ordered nanophotonic devices by preform-to-fiber fabrication. IEEE Journal of Selected Topics in Quantum Electronics 12(6):1077-1213. Bayindir, Mehmet, Fabien Sorin, Ayman F. Abouraddy, Jeff Viens, Shandon D. Hart, John D. Joannopoulos, and Yoel Fink. 2004. Metal-insulator-semiconductor optoelectronic fibres. Nature 431(7010):826-829. Bennett, Drake. 2007. Environmental defense: Increasingly, the military sees energy efficiency—and moving away from oil—as part of its national security mission. Does that mean the Pentagon is turning green? Boston Globe, May 27. Available at http://www.boston.com/news/education/higher/articles/2007/05/27/environmental_defense/. Bennett, C.H., and G. Brassard. 1984. Quantum cryptography: Public key distribution and coin tossing. Pp. 175-179 in IEEE International Conference on Computers Systems and Signal Processing, Bangalore, India: IEEE. Bennett, C.H., F. Bessette, G. Brassard, L. Salvail, and J. Smolin. 1991. Experimental quantum cryptography. Lecture Notes in Computer Science 473:253-265. Bennett, Charles H., and Stephen J. Wiesner. 1992. Communication via one- and two-particle operators on Einstein-Podolsky-Rosen states. Physical Review Letters 69(20):2881. Cirac, J.I., and P. Zoller. 1995. Quantum computations with cold trapped ions. Physical Review Letters 74(20):4091-4094. Deutsch, D. 1985. Quantum theory, the Church-Turing principle and the universal quantum computer. Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences (1934-1990), 400(1818):97-117. DiVincenzo, David P. 2000. The physical implementation of quantum computation. Forschritte Der Physik 48(9-11):771-783. D-Wave Systems. 2007. World’s First Commercial Quantum Computer Demonstrated 2007. Available at http://www.dwavesys.com/index.php?mact=News,cntnt01,detail,0&cntnt01articleid=4&cntnt01origid=15&cntnt01returnid=21. Accessed May 28, 2007. Feynman, R.P. 1982. Simulating physics with computers. International Journal of Theoretical Physics 21(6-7):467-488. Feynman, R.P. 1984. Quantum-mechanical computers. Journal of the Optical Society of America B-Optical Physics 1(3):464-464. Feynman, R.P. 1986. Quantum-mechanical computers. Foundations of Physics 16(6):507-531. FLIR Systems. 2007. Airborne Systems. Available at http://www.flir.com/imaging/Airborne/index.aspx. Accessed May 28, 2007. Gershenfeld, N., and I. Chuang. 1997. The usefulness of NMR quantum computing—Response. Science 277(5332):1689-1690. Grover, Lov K. 1997. Quantum mechanics helps in searching for a needle in a haystack. Physical Review Letters 79(2):325-328. Hattori, Haroldo, Christian Seassal, Xavier Letartre, Pedro Rojo-Romeo, Jean Leclercq, Pierre Viktorovitch, Marc Zussy, Lea di Cioccio, Loubna El Melhaoui, and Jean-Marc Fedeli. 2005. Coupling analysis of heterogeneous integrated InP based photonic crystal triangular lattice band-edge lasers and silicon waveguides. Optics Express 13(9):3310-3322. Hughes, R.J., W.T. Buttler, P.G. Kwiat, S.K. Lamoreaux, G.L. Morgan, J.E. Nordholt, and C.G. Peterson. 2000. Free-space quantum key distribution in daylight. Journal of Modern Optics 47(2-3):549-562. id Quantique. 2007.Vectis Link Encryptor. Available at http://www.idquantique.com/products/vectis.htm. Accessed May 28, 2007. Imamoglu, A., D.D. Awschalom, G. Burkard, D.P. DiVincenzo, D. Loss, M. Sherwin, and A. Small. 1999. Quantum information processing using quantum dot spins and cavity QED. Physical Review Letters 83(20):4204-4207.
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